A large study of heterosexual and gay couples saw no cases of linked HIV transmission during 44,500 condomless sex acts when the positive partner was on combination antiretroviral therapy (ART) with suppressed viral load, according to a presentation at the 21st Conference on Retroviruses and Opportunistic Infections (CROI 2014) this week in Boston. Based on these findings, there is no reason for legal action against people not using condoms while on effective ART.

Jens Lundgren from the University of Copenhagen reported the first findings from the PARTNER study, looking at HIV transmission within more than 750 serodiscordant couples -- together contributing nearly 900 years of follow-up -- who reported not using condoms.

While this much condomless vaginal and anal sex would be expected to lead to 50 to 100 transmissions, based on historical data, the researchers did not see any cases of linked transmission in which newly acquired HIV matched a partner's strain.

While the risk of transmission per sex act while on ART fell to nearly zero, the cumulative risk over time must still be considered. Lundgren said. Estimates of risk over 10 years may reach as high as 1 in 25 overall, or up to 1 in 10 for anal sex. Whether this level of risk is acceptable "is not for us to say, but for people to decide," he said.

Lundgren emphasized, however, that there is "no reasonable legal action you can take against people for not using condoms" if they are on effective ART, and no indication for post-exposure prophylaxis (PEP) in this situation.

These findings support the recent decision by the Centers for Disease Control and Prevention (CDC) -- at the urging of activists -- to refer to "condomless sex" rather than "unprotected sex," recognizing that there are other effective prevention approaches besides condom use.